The processes related to drilling and casing earth boreholes are well known. Earth boreholes are produced through the use of pipe strings known as drill strings. The boreholes, once drilled, are cased with a different form of pipe string cemented into the earth. Yet another pipe string is commonly suspended inside the casing to conduct fluids produced far below to the earth surface. The pipe strings are interrelated in many respects, and it is important that drilling and well completion people have the lengths of pipe rather accurately measured as an assembly in each case. In the past, the measuring art has yielded many errors with serious economic and safety consequences.
At the present time, there are two general classifications of pipe used in earth borehole operations, primarily in oil and gas well drilling and oil and gas production:
(1) shouldered pipe joints used in drilling; and PA0 (2) non-shouldered pipe joints used in casing and in production tubing. PA0 (1) mechanical indentations; PA0 (2) surface painting; PA0 (3) installation of stainless steel or plastic banding materials which have been premarked with the coded information; PA0 (4) installation of flexible labels with high strength epoxy; PA0 (5) indentations made by electro-magnetic beam. PA0 (1) holographic techniques (laser-barcode readers); PA0 (2) optical emission; PA0 (3) mechanical detection;
Shouldered pipe is used in well drilling and added to the drill string joint-by-joint as the well is deepened by drilling. During drilling activity, the drill string is occasionally removed from the well to change drill bits and other components, usually near the bit. During these "round trips," the drill string is usually separated in three joint "stands" and set back on the rig floor in a vertical stance to await reassembly in reverse order, as the drill string is once more assembled into the well to resume drilling.
During the removal phase of the "round trip," the driller and others remain alert for any signs of damage on any emerging pipe joint or its connections. Any damaged joint will be removed and "laid down," so that it will not be later reassembled into the drill string. The total length of all such laid down joints must be made up by the addition of new pipe joints. The new joints will be pulled up to the rig floor from the pipe racks at the rig site.
From the beginning of the assembly, the pipe joint drawn up to the rig floor will be "strapped." This is an act of measuring the shoulder-to-shoulder length, usually with a steel measuring tape. The driller maintains a pipe tally of all lengths added to the drill string. Since there may eventually be in the order of five hundred drill string joints and shorter reamers and stabilizers, as well as jars and downhole motors, the tally is necessarily large. With the occasional laid down joints and new additions, the volumnous tally is often modified. Some of these tally changes are interrupted by occasional shift changes of all rig floor personnel. Some confusion is inevitable.
An entire production zone can be erroneously logged in terms of distance below the earth surface due to errors in pipe length. Subsequent completion operations such as perforating can miss the production zone. To reduce this likelihood, considerable expense is incurred in logging wells independently of drill strings.
When non-shouldered pipe is installed in a well, a different set of problems arises. Non-shouldered pipe has a variable thread make-up responsive to connection geometry and makeup torque applied. The total joint length is measured and thread make-up allowance is subtracted. The problem of continual round tripping related to the drill string does not occur with non-shouldered pipe.
In addition to the errors cited above, pipe strings suspended in great length stretch several feet due to axial strain from load. With drill string, drillers use prepared tables to indicate stretch to be expected. In drilling, the bit load applied at the bottom changes drill string total length. Changes in pressure applied to the drill pipe to force drilling fluids to circulate also change pipe length.